The present program is aimed at delineating mechanisms underlying our perception of the spatial location of objects and of ourselves. The particular concern is with the determination of visually perceived direction by extraretinal information regarding eye position, by the structure and content of the visual field, and by the interaction of both. Psychophysical measurements will be made of (1) matches in the horizontal location of visual and auditory targets, (2) the visual location of the eye-level horizontal, and (3) the visual location of the body's median plane. We will investigate whether the large visual-context dependent and gaze-dependent errors (less than 20 degrees) in these three measures which we find in partially paralyzed individuals are based on faulty extraretinal information as our previous work has suggested. We will investigate whether smaller Type A and Type B errors in auditory/visual matches in normal individuals are based on auditory and extraretinal deficits respectively. We will search for these deficits in individuals with myasthenia gravis (where we expect them) and in individuals with marked phorias (where they may occur), and if they are found we will develop simplified quantitative versions of our measurement procedures for use in diagnosis and measurement of ophthalmoplegias for clinical use.